The present invention pertains to certain cold water soluble methylated, ethylated or carboxymethylated non-gelatinized granular starch derivatives; to processes for the preparation of such starch derivatives and to the use thereof in thickening latex paint compositions.
Methylated, ethylated and carboxymethylated starches are generally known materials which can be prepared by reacting a starch starting material with, respectively, a methylating agent such as dimethyl sulfate or a methyl halide, an ethylating agent such as diethyl sulfate or an ethyl halide or a carboxymethylating agent such as a haloacetic acid or a salt thereof under aqueous alkaline reaction conditions.
U.S. Pat. No. 1,444,257 to Lillenfeld (issued Feb. 6, 1923) discusses the usefulness of ethylated starch and methylated starch as protective colloids and, according to the book entitled "Starch: Chemistry and Technology, Volume 11, Industrial Aspects" by Whistler and Paschall (1967) at page 314, such starch derivatives have also been suggested as being useful as soil suspending agents in detergents, as flocculants for pigments, thickeners in paper screening and as a component of photographic transfer layers.
A book entitled Starch Production Technology by J. A. Radley, Applied Science Publishers, Ltd. (1976) indicates, at page 506, that carboxymethyl starch derivatives are much used in the textile industry. Such book also reports that conducting the reaction to prepare such derivatives in a water-miscible solvent (preferably isopropanol) containing minor amounts of water improves the reaction efficiency.
U.S. Pat. No. 3,769,248 to Kovats (issued Oct. 30, 1973) mentions carboxymethyl starch as being suitable for use as a protective colloid in vinyl acetate emulsion polymerization and copolymerization processes.
At page 319 of the aforementioned book by Whistler and Paschall, it is noted that miscellaneous uses of carboxymethyl starch include use as an indicator in iodometric titrations, use as a soil conditioner, use as a component in latex paint, use as a paint remover component, use as a binder for kaolin castings as well as usage in lithography applications, photographic film emulsions and granular explosive compositions.
In the manufacture of methylated, ethylated and carboxymethylated starch derivatives via the reaction, respectively, of starch with methyl halides, dimethyl sulfate, diethyl sulfate, ethyl halides, haloacetic acids (or salts thereof), etc. under aqueous alkaline conditions, inorganic salts such as alkali metal (e.g., sodium or potassium) halides or sulfates are generated as reaction byproducts. On page 507 of the aforementioned book by J. A. Radley, it is stated that attempts to remove such inorganic salts from carboxymethyl starch products have included (a) precipitation and washing with alcohol; (b) salting out with sodium sulfate and screening off the salts after grinding; and (c) precipitation with alum and resolubilizing with alkali. This reference goes on to note, however, that for the majority of commercial applications, the salts are left in since their technical effect is one of dilution only.